We have enrolled 48 patients in the study, "Evaluation of Toxicity of Inhaled Nitric Oxide in Neonates with Persistent Pulmonary Hypertension". Forty-one infants had PPHN as a result of medical illnesses, such as sepsis or meconium aspiration syndrome. Thirty-two of these children had a beneficial response to nitric oxide at 20 ppm after 1 hour, and only 3 of these infants went on to require ECMO. Eight infants in this group did not respond to inhaled nitric oxide, of whom six progressed to the point where they required ECMO. All infants who progressed to ECMO survived. One infant who responded to nitric oxide died, after support was withdrawn because the infant was diagnosed with a severe chromosomal abnormality. The remaining seven infants in the study had PPHN as a result of congenital diaphragmatic hernia. Two of the seven had had fetal surgery with tracheal clip placement, and were delivered prematurely. Both of the fetal surgery infants had a beneficial response to nitric oxide with improvement in oxygenation. One infant survived, and one had support withdrawn because of poor neurologic prognosis. The other 5 infants with PPHN secondary to congenital diaphragmatic hernia did not respond to inhaled nitric oxide, and all required ECMO. Three of these 5 infants died of severe pulmonary hypoplasia. None of the infants with PPHN who received inhaled nitric oxide have shown an elevation in their plasma nitrotyrosine levels, a marker of nitric oxide derived oxidant stress.